Analog-to-digital converters are pervasively used in many applications ranging from DC and low frequency sensor applications, up to high frequency A/D converters used for wire-line and wireless communications. One commonly used A/D architecture is an oversampled A/D converter such as a sigma-delta modulator, which is generally a feedback loop that includes one or more integrators in a forward path followed by a low-resolution quantizer, the output of which is subtracted from the input to form an error signal. The quantizer output, which can have as low as one bit of resolution, is then decimated using a digital decimator to produce a multi-bit output.
One of the well-known properties of the sigma-delta modulator is that the quantization noise of the modulator is shaped by the loop filter. This noise shaping can yield a vast improvement in signal-to-noise ratio. For example, in a PCM A/D converter, each doubling in frequency yields a 3 dB improvement in the SNR. In a sigma-delta converter, however, each doubling in frequency ideally improves the SNR of the A/D converter by approximately, (6L+3) dB, where L is the order of the sigma-delta modulator. As such, sigma-delta modulators can provide SNRs of well over 100 dB for audio and low frequency applications.
One common way of implementing a sigma-delta A/D converter is by using a switched capacitor circuit. At its most basic level, switched capacitor circuits perform analog signal processing in the charge domain by sampling charge on capacitors. CMOS processes are particularly suited for such circuits. By using a combination of feedback amplifiers, switching devices and ratioed capacitors, various sampled analog transfer functions, including the integrator used for sigma-delta modulators may be accurately represented even in the presence of a high amount of absolute component value variation.
For very low frequency and quasi-DC applications, however, the DC offset of CMOS amplifiers, as well as the flicker noise present in CMOS devices pose some design challenges to very low frequency and quasi-DC applications.